The present invention relates to a rotary atomizing electrostatic applicator and a shaping air ring for the applicator.
High quality is required of automotive body painting, which is connected directly to design and marketability of the automobile. An electrostatic applicator has long been adopted for automotive body painting. The electrostatic applicator continues evolving to answer demands of the automotive industry. The demands roughly fall into two categories. One of the categories asks for further reduction in amounts of wasted paint, i.e., further improvement of coating efficiency. The other category asks for quality improvement of painting. In conventional approaches to quality improvement of metallic painting regarded as important in the quality improvement of painting, a technique which uses strong shaping air has been adopted for many years.
The applicator adapted most often in the automotive industry is a rotary atomizing electrostatic applicator equipped with a cup shaped rotary atomizing head called a “bell cup.” Hereinafter the rotary atomizing head will be referred to as a “bell cup.” A basic idea about atomization in the rotary atomizing electrostatic applicator has already been established. The idea is based on Equation 1 below.P3=A×(Qμ/ρN2r2)  [Equation 1]
where
P: Diameter of paint particle (mm)
A: Coefficient
Q: Feed rate of paint, i.e., amount of paint fed to bell cup (cc/min)
μ: Viscosity (Cp) of paint
ρ: Specific gravity of paint
N: Rotational speed of bell cup (rpm)
r: Radius of bell cup
The following can be seen from Equation 1 above. That is, paint particle diameter P is proportional to the amount Q of paint fed to the bell cup, i.e., the paint discharge rate of the applicator. In other words, Equation 1 teaches that the paint particle diameter P increases with increases in the paint discharge rate.
Next, volume V of a paint particle is given by Equation 2 below.V=(4/3)×π×(P/2)3=(1/6)πP3  [Equation 2]
Substituting Equation 1 into Equation 2 yields Equation 3 below.V=(π/6)×A×Q×μ×(1/ρN2r2)  [Equation 3]
In Equation 3, {(π/6)×A} is a constant. When {(π/6)×A} is substituted with “B,” Equation 3 can be expressed by Equation 4 below.V=(B×Q×μ)/(ρN2r2)  [Equation 4]
The following can be seen from Equation 4. That is, the volume V of the paint particle is inversely proportional to the square of the rotational speed (bell revolution) N of the bell cup. The volume V of the paint particle is also inversely proportional to the square of the radius r of the bell cup. In other words, Equation 4 teaches that increasing the rotational speed N of the bell cup is effective in decreasing the volume V of the paint particle. Also, Equation 4 teaches that increasing the radius r of the bell cup is effective in decreasing the volume V of the paint particle.
Based on instructions given by Equations 1 and 4, a technique which involves increasing the rotational speed of the bell cup and/or increasing the radius of the bell cup has conventionally been adopted as a technique for increasing atomization, i.e., decreasing the paint particle size.
It is known that to improve the quality of metallic painting, the velocity of collision of paint particles with automotive body surface can be increased. Based on this idea, an electrostatic applicator applicable to metallic painting has been developed. The electrostatic applicator is called a “metal bell” in the industry (Japanese Patent Laid-Open No. 3-101858).
The metal bell adopts a configuration in which the shaping air is directed at the back or outer circumferential edge of the bell cup. The shaping air of the metal bell is assigned two roles: the role of (a) atomizing the paint and (b) directing the paint particles at a workpiece and defining a painting pattern. To enhance the function (b) of defining the painting pattern, an electrostatic applicator has been developed which twists the shaping air in a direction opposite to the rotation direction of the bell cup (Japanese Patent Laid-Open No. 2012-115736). Japanese Patent Laid-Open No. 2012-115736 proposes to control a painting pattern width by discharging additional shaping air forward on a radially outer side of the shaping air while controlling discharge pressure or flow rate of the additional shaping air.
Incidentally, a painting process in which the electrostatic applicator is installed makes up part of an automotive production line. That is, the automotive production line includes a pressing process, a welding process, the painting process, and an assembly process.
Currently, the electrostatic applicator installed in the automotive production line is operated using, for example, the following parameters.
(i) Rotational speed of the bell cup: 20,000 to 30,000 rpm
(ii) Paint discharge rate: 200 to 300 cc/min
(iii) Twist angle of shaping air: 30 to 45 degrees
(iv) Diameter of bell cup: 77 mm
(v) Discharge pressure of shaping air: 0.10 to 0.15 MPa
(vi) Flow rate of shaping air; 500 to 650 NL/min
(vii) Painting pattern width: 300 to 350 mm in diameter
(viii) Coating efficiency: approximately 60 to 70%
Here, the above-mentioned twist angle of shaping air means the twist angle of the shaping air directed at the back or outer circumferential edge of the bell cup.
In the case of metallic painting, which uses strong shaping air (0.20 MPa, 650 NL/min), the coating efficiency is approximately 10% lower than non-metallic, i.e., solid painting. The painting pattern width is approximately 320 mm in diameter.
Note that the diameter of the bell cup is 70 mm or 65 mm depending on the applicator maker. The bell cups of these sizes are used to paint outer plates of automotive bodies. To paint bumpers or small parts, an electrostatic applicator equipped with a bell cup of 30 mm, 40 mm, or 50 mm in diameter is used. The rotational speed of the bell cup may be higher than 30,000 rpm.
When the amount of paint discharged by the electrostatic applicator is increased, it is necessary to keep film thickness constant by increasing the coating speed. For example, when the paint discharge rate is doubled compared to a conventional one, if the film thickness is kept at a conventional level by doubling the coating speed, the number of applicators can be reduced. In other words, if the same number of applicators as before is used, the time required for the painting process can be reduced. Therefore, if the paint discharge rate of the electrostatic applicator can be increased from, for example, the current level of 200 to 300 cc/min to, for example, 500 cc/min or 1,000 cc/min, this can contribute greatly to improvement in the production capacity of the automotive production line. However, things are not so simple as to be able to merely increase the paint discharge rate of the rotary atomizing electrostatic applicator. Increasing the paint discharge rate increases the diameter of the paint particles, making it difficult to maintain painting quality. That is, the paint discharge rate and painting quality are in a trade-off relation to each other.
The problem of the trade-off causes the following problems when a conventional technique is adopted for atomization of paint. The conventional technique involves increasing the rotational speed of the bell cup (bell revolution) and/or the diameter of the bell cup based on the instructions given by Equations 1 and 4 described above.
(1) Problems Involved in Setting the Rotational Speed of the Bell Cup High:
(1-1) Reduction in Coating Efficiency:
A centrifugal force acts on the paint particles flying out of the rotating bell cup. The centrifugal force increases with increases in the rotational speed. With increases in the centrifugal force, it becomes increasingly necessary to raise the discharge pressure or flow rate of shaping air in order to deflect the paint particles toward the workpiece against the centrifugal force. However, if the shaping air is intensified, the paint particles hit a workpiece surface at higher velocity and the shaping air bounces off the workpiece. As the shaping air bounces off, the paint particles are blown off before attaching to the workpiece surface. Thus, there is a problem in that intensifying the shaping air leads to a fall in the coating efficiency.
(1-2) Double Pattern:
If the shaping air is intensified, the painting pattern is prone to be doubled. The double pattern refers to a condition in which due to differences in the weight of paint particles, small paint particles (light particles) gather in a center portion of the painting pattern while large paint particles (heavy particles) gather in an outer circumferential part. When a double painting pattern is produced, a paint film tends to become relatively thick in the center portion and relatively thin in the outer circumferential portion. Consequently, with the double painting pattern, there is a problem in that paint film thickness is prone to become ununiform.
(2) Problems with a Large-Diameter Bell Cup:
(2-1) Overspray:
Adoption of a large-diameter bell cup increases the painting pattern width, i.e., painting pattern diameter. When the painting pattern width is increased, in order to implement a painted surface of uniform film thickness in forming a paint film, for example, by reciprocating motion of the applicator, it is necessary to overspray half the circular painting pattern. This means increases in the amount of paint wasted by the overspray.
(2-2) Centrifugal Force Acting on Paint Particles:
At equal rotational speed, a bell cup with a large radius has a higher circumferential velocity than a bell cup with a small radius. Thus, when a bell cup with a large radius is adopted, a large centrifugal force acts on the paint particles flying out of the bell cup. The problems encountered when a large centrifugal force acts on paint particles are as described above.